The duck started off well, then became more like a turkey and finallya four-legged abstraction.

Scientists studying a female patient with semantic dementia (a form of dementia where patients lose the ability to recognize the name of common objects and their significance) asked her to draw a duck:

When the original picture of a duck was in front of her, she sketched an excellent copy, showing that she was perfectly capable of seeing correctly and taking in information, but when asked to wait and draw from memory, a very different effect was seen.

With a ten-second delay, she gave the duck a turkey-like head, a mouth and an eyebrow, and started to draw a third leg before remembering that the bird had only two. With a 60-second delay, the duck became a four-legged abstraction with a wild tail, a more pronounced eyebrow and a smile.

Matthew Lambon-Ralph, who conducted the study, said: “The initial copy was very good, but with a delay you have to rely on recalling the meaning of what you have seen. As understanding is distorted, confusion sets in. Birds start to be confused with other animals, and that’s when you get a four-legged duck.”

Look at stuffed toy ducks around Easter. In the last few years it has become more and more common for their wings to lose their winglike nature. There are far too many plush ducks that are beginning to look like quadrupeds, as if they were bright yellow platypuses.